High-Bandwidth Multimode Optical Fiber with Reduced Cladding Effect

ABSTRACT

The present invention embraces an optical fiber that includes a central core having an alpha refractive index profile with respect to an outer cladding. The optical fiber also includes an inner cladding, a depressed trench, and an outer cladding. The optical fiber achieves reduced bending losses and a high bandwidth with a reduced cladding effect for high-data-rate applications.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application claims the benefit of commonly assigned pending French application Ser. No. 09/58381 for a “Fibre Optique Multimode à Très Large Bande Passante avec une Interface C≦ur-Gaine Optimiséé” (filed Nov. 25, 2009, at the National Institute of Industrial Property (France)).

This application further claims the benefit of commonly assigned U.S. Patent Application No. 61/265,101 for a “Fibre Optique Multimode à Très Large Bande Passante avec une Interface Cœur-Gaine Optimiséé” (filed Nov. 30, 2009).

Each of the foregoing commonly assigned patent applications is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of fiber optic transmission, and, more specifically, to a multimode optical fiber having reduced bending losses and a high bandwidth for high data rate applications.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core n_(c) is typically greater than the refractive index of the optical cladding n_(g) (i.e., n_(c)>n_(g)).

For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.

Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode optical fiber, the higher order modes are strongly attenuated. The typical diameter of a single-mode or multimode glass fiber is 125 microns. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems, because multimode light sources, connectors, and maintenance can be obtained at a lower cost.

Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard is hereby incorporated by reference in its entirety.

In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 meters). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode optical fibers has been reduced from 62.5 microns to 50 microns.

Typically, an optical fiber should have the broadest possible bandwidth to perform well in a high-bandwidth application. For a given wavelength, the bandwidth of an optical fiber may be characterized in several different ways. Typically, a distinction is made between the so-called “overfilled launch” condition (OFL) bandwidth and the so-called “effective modal bandwidth” condition (EMB). The acquisition of the OFL bandwidth assumes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (e.g., using a laser diode or light emitting diode (LED)).

Recently developed light sources used in high-bandwidth applications, such as VCSELs (Vertical Cavity Surface Emitting Lasers), exhibit an inhomogeneous excitation over the radial surface of the optical fiber. For this kind of light source, the OFL bandwidth is a less suitable measurement, and so it is preferable to use the effective modal bandwidth (EMB). The calculated effective bandwidth (EMBc) estimates the minimum EMB of a multimode optical fiber independent of the kind of VCSEL used. The EMBc is obtained from a differential-mode-delay (DMD) measurement (e.g., as set forth in the FOTP-220 standard).

An exemplary method of measuring DMD and calculating the effective modal bandwidth can be found in the FOTP-220 standard, which is hereby incorporated by reference in its entirety. Further details on this technique are set forth in the following publications, each of which is hereby incorporated by reference: P. F. Kolesar and D. J. Mazzarese, “Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications,” Proceedings of the 51st International Wire and Cable Symposium, pp. 453-460; and D. Coleman and Philip Bell, “Calculated EMB Enhances 10 GbE Performance Reliability for Laser-Optimized 50/125 μm Multimode Fiber,” Corning Cable Systems Whitepaper.

FIG. 1 shows a schematic diagram of a DMD measurement according to the criteria of the FOTP-220 standard as published in its TIA SCFO-6.6 version of Nov. 22, 2002. FIG. 1 schematically represents a part of an optical fiber (i.e., an optical core surrounded by an outer cladding). A DMD graph is obtained by successively injecting into the multimode optical fiber a light pulse having a given wavelength λ₀ with a radial offset between each successive pulse. The delay of each pulse is then measured after a given length of fiber L. Multiple identical light pulses (i.e., light pulses having the same amplitude, wavelength, and frequency) are injected with different radial offsets with respect to the center of the multimode optical fiber's core. The injected light pulse is depicted in FIG. 1 as a black dot on the optical core of the optical fiber. In order to characterize an optical fiber with a 50-micron diameter, the FOTP-220 standard recommends that at least 24 individual measurements be carried out (i.e., at 24 different radial offset values). From these measurements, it is possible to determine the modal dispersion and the calculated effective modal bandwidth (EMBc).

The TIA-492AAAC-A standard, which is hereby incorporated by reference in its entirety, specifies the performance requirements for 50-micron-diameter multimode optical fibers used over long distances in Ethernet high-bandwidth transmission network applications. The OM3 standard requires, at a wavelength of 850 nanometers, an EMB of at least 2,000 MHz·km. The OM3 standard assures error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4,700 MHz·km to obtain error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 550 meters. The OM4 standard is hereby incorporated by reference in its entirety.

In a multimode optical fiber, the difference between the propagation times, or group delay times, of the several modes along the optical fiber determine the bandwidth of the optical fiber. In particular, for the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag between the pulses propagating along different radial offsets of the optical fiber.

For example, as shown in the graph on the right side of FIG. 1, a time lag is observed between the individual pulses. This FIG. 1 graph depicts each individual pulse in accordance with its radial offset in microns (y-axis) and the time in nanoseconds (x-axis) the pulse took to pass along a given length of the optical fiber.

As depicted in FIG. 1, the location of the peaks along the x-axis varies, which indicates a time lag (i.e., a delay) between the individual pulses. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse and reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is directly linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.

To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.

A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:

$n = {n_{1}\sqrt{1 - {2\; {\Delta \left( \frac{r}{a} \right)}^{\alpha}}}}$

wherein,

α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\Delta = \frac{\left( {n_{1}^{2} - n_{0}^{2}} \right)}{2\; n_{1}^{2}}$

where n₀ is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.

In practice, however, a manufactured multimode optical fiber has a graded-index central core surrounded by an outer cladding of constant refractive index. The core-cladding interface interrupts the core's alpha-index profile. Consequently, the multimode optical fiber's core never corresponds to a theoretically perfect alpha profile (i.e., the alpha set profile). The outer cladding accelerates the higher-order modes with respect to the lower-order modes. This phenomenon is known as the “cladding effect.” In DMD measurements, the responses acquired for the highest radial positions (i.e., nearest the outer cladding) exhibit multiple pulses, which results in a temporal spreading of the response signal. Therefore, bandwidth is diminished by this cladding effect.

Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local area networks (LANs). In such applications, the optical fibers may be subjected to accidental or otherwise unintended bending, which can modify the mode power distribution and the bandwidth of the optical fiber.

It is therefore desirable to achieve multimode optical fibers that are unaffected by bends having a radius of curvature of less than 10 millimeters. One proposed solution involves adding a depressed trench between the core and the cladding. Nevertheless, the position and the depth of the trench can significantly affect the optical fiber's bandwidth.

Japanese Patent Publication No. JP 2006/47719 A, which is hereby incorporated by reference in its entirety, discloses a graded index optical fiber having a depressed trench in its cladding. Nevertheless, the disclosed optical fiber exhibits higher bending losses than desired and a relatively low bandwidth. Moreover, the disclosed optical fiber's cladding effect is not mentioned.

International Publication No. WO-A-2008/085851, which is hereby incorporated by reference in its entirety, discloses a graded-index optical fiber having a depressed trench in its cladding. Nevertheless, the disclosed optical fiber exhibits a relatively low bandwidth, and its cladding effect is not mentioned.

U.S. Patent Application Publication No. 2009/0154888, which is hereby incorporated by reference in its entirety, discloses a graded-index fiber having a depressed trench in its cladding. Nevertheless, the disclosed fiber exhibits a relatively low bandwidth, and its cladding effect is not mentioned.

European Patent No. EP 0131729, which is hereby incorporated by reference in its entirety, discloses a graded-index optical fiber having a depressed trench in its cladding. According to this document, to achieve a high bandwidth, the distance between the end of the graded-index profile core and the beginning of the depressed trench should be between 0.5 micron and 2 microns. Nevertheless, the disclosed optical fiber exhibits higher bending losses than desired. Moreover, the disclosed optical fiber's cladding effect is not mentioned.

Therefore, a need exists for a graded-index multimode optical fiber having reduced bending losses and a high bandwidth with a reduced cladding effect for high-data-rate applications.

SUMMARY

In one aspect, the present invention embraces a multimode optical fiber that includes a central core surrounded by an outer cladding. The central core has a radius r₁ and an alpha-index profile with respect to the outer cladding. An inner cladding is positioned between the central core and the outer cladding (e.g., immediately surrounding the central core). The inner cladding has (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to the outer cladding. A depressed trench is positioned between the inner cladding and the outer cladding (e.g., immediately surrounding the inner cladding). The depressed trench has (i) an outer radius r_(t), (ii) a width w_(t), and (iii) a refractive index difference Δn_(t) with respect to the outer cladding. Typically, the width w₂ of the inner cladding and the refractive index difference Δn_(t) between the depressed trench and the outer cladding satisfy the following inequality:

${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{100 \times \Delta \; n_{t}} - 0.4} + 2.}$

In an exemplary embodiment, the alpha-index profile of the central core has an alpha parameter α of between about 1.9 and 2.1 (e.g., between about 2.04 and 2.1).

In yet another exemplary embodiment, the refractive index difference between the central core and the outer cladding has a maximum value Δn₁ of between about 11×10⁻³ and 16×10⁻³.

In yet another exemplary embodiment, the refractive index difference Δn₂ between the inner cladding and the outer cladding is between about −0.05×10⁻³ and 0.05×10⁻³.

In yet another exemplary embodiment, the refractive index difference Δn_(t) between the depressed trench and the outer cladding is between about −15×10⁻³ and −3×10⁻³.

In yet another exemplary embodiment, the width w_(t) of the depressed trench is between about three microns and five microns.

In one particular embodiment, the depressed trench has a volume v_(t) of between about 200%·μm² and 1,200%·μm² (e.g., between about 250%·μm² and 750%·μm²).

In yet another exemplary embodiment, for two turns around a bend radius of 15 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.1 dB (e.g., less than about 0.05 dB).

In yet another exemplary embodiment, for two turns around a bend radius of 10 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.3 dB (e.g., less than about 0.1 dB).

In yet another exemplary embodiment, for two turns around a bend radius of 7.5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.4 dB (e.g., less than about 0.2 dB).

In yet another exemplary embodiment, for two turns around a bend radius of 5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 1 dB (e.g., less than about 0.3 dB).

In another particular embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 5,000 MHz·km, such as at least about 10,000 MHz·km (e.g., at least about 15,000 MHz·km).

In yet another particular embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of typically at least about 1,500 MHz·km (e.g., 3,500 MHz·km or more), more typically at least about 5,000 MHz·km (e.g., at least about 10,000 MHz·km).

In yet another exemplary embodiment, the multimode optical fiber has a numerical aperture of 0.2±0.015 (i.e., between 0.185 and 0.215).

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber typically has an outer DMD value of less than about 0.33 ps/m, more typically less than about 0.25 ps/m (e.g., less than about 0.14 ps/m).

In another aspect, the present invention embraces an optical-fiber system that includes at least a portion of one of the present multimode optical fibers. The optical-fiber system typically has a data rate of at least about 10 Gb/s over a distance of about 100 meters. In one embodiment, the optical-fiber system has a data rate of at least about 10 Gb/s over a distance of about 300 meters.

The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the invention, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an exemplary DMD measurement method and graph.

FIG. 2 graphically depicts the refractive index profile of an exemplary optical fiber according to the present invention.

FIG. 3 graphically depicts the bandwidth at a radial offset of 24 microns (ROB24) as a function of the width of an inner cladding and a depressed trench's refractive index difference with respect to an outer cladding.

FIG. 4 graphically depicts the bandwidth at a radial offset of 24 microns (ROB24) as a function of the alpha parameter of the central core's alpha-index profile.

FIG. 5 graphically depicts the overfilled launch condition (OFL) bandwidth as a function of the alpha parameter of the central core's alpha-index profile.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber that achieves reduced bending losses and a high bandwidth with a reduced cladding effect for high-data rate applications.

FIG. 2 depicts the refractive index profile of an exemplary optical fiber in accordance with the present invention. The optical fiber includes a central core having an outer radius r₁ and an alpha refractive index profile (i.e., an alpha-index profile) with respect to an outer cladding (e.g., an outer optical cladding) surrounding the central core. Typically, the core has a radius r₁ of about 25 microns. The refractive index difference between the central core and the outer cladding typically has a maximum value Δn₁ of between about 11×10⁻³ and 16×10⁻³ (e.g., between about 12×10⁻³ and 15×10⁻³). The central core typically has an alpha profile with an alpha parameter of between about 1.9 and 2.1. In a particular embodiment, the central core has an alpha profile with an alpha parameter of between about 2.04 and 2.1, such as between about 2.06 and 2.08 (e.g., between about 2.07 and 2.08). In another particular embodiment, the central core has an alpha profile with an alpha parameter of between about 2.05 and 2.08 (e.g., between about 2.06 and 2.07).

For reasons of cost, the outer cladding is typically made of natural silica, but it may alternatively be made of doped silica.

The optical fiber includes an inner cladding positioned between the central core and the outer cladding. In one embodiment, the inner cladding immediately surrounds the central core. The inner cladding has an outer radius r₂, a width w₂, and a refractive index difference Δn₂ with respect to the outer cladding. The refractive index difference Δn₂ between the inner cladding and the outer cladding is typically between about −0.05×10⁻³ and 0.05×10⁻³. More typically, and as illustrated in FIG. 2, the refractive index difference Δn₂ between the inner cladding and the outer cladding is approximately zero. The characteristics of the inner cladding facilitate the achievement of high bandwidths.

The fiber typically includes a depressed trench positioned between the inner cladding and the outer cladding. By way of example, the depressed trench may immediately surround the inner cladding. The depressed trench has an outer radius r_(t), a width w_(t), and a refractive index difference Δn_(t) with respect to the outer cladding. The width w_(t) of the depressed trench is typically between about 3 and 5 microns (μm).

Typically, the term “depressed trench” is used to describe a radial portion of an optical fiber that has a refractive index that is substantially less than the refractive index of the outer cladding. In this regard, the refractive index difference Δn_(t) between the depressed trench and the outer cladding is typically between about −15×10⁻³ and −3×10⁻³, more typically between about −10×10⁻³ and −5×10⁻³.

Generally speaking, a refractive index difference with respect to the outer cladding can also be expressed as a percentage using the following equation:

${\Delta \; \% (r)} = \frac{100 \times \left( {{n(r)}^{2} - n_{cladding}^{2}} \right)}{2\; {n(r)}^{2}}$

where n(r) is the comparative refractive index value as a function of radial position (e.g., the refractive index n_(t) of the depressed trench), and n_(cladding) is the refractive index value of the outer cladding. Those of ordinary skill in the art will appreciate that this equation can be used if the refractive index varies over a given section of the optical fiber (i.e., the refractive index value varies as a function of radial position) or if the refractive index is constant over a given section.

Those of ordinary skill in the art will appreciate that the outer cladding typically has a constant refractive index.

That said, if the outer cladding has a non-constant refractive index, the refractive index difference (e.g., Δn₁, Δn₂, Δn_(t), Δ₁%, Δ₂%, or Δ_(t)%) between a section of the optical fiber and the outer cladding is measured using the refractive index of the inner most portion of the outer cladding (i.e., the portion of the outer cladding that is expected to most strongly affect the propagation of optical signals within the optical fiber).

A constant refractive index difference with respect to the outer cladding can also be expressed as a percentage by the following equation:

${\Delta \; \%} = \frac{100 \times \left( {n^{2} - n_{cladding}^{2}} \right)}{2\; n^{2}}$

where n is the comparative refractive index (e.g., the refractive index n_(t) of the depressed trench), and n_(cladding) is the refractive index of the outer cladding.

As used herein, the depressed trench's volume v_(t) is defined by the following equation:

v_(t) = 2 π × ∫_(r_(int))^(r_(t))Δ_(t)  % (r) × r × r

in which Δ_(t)% (r) is the depressed trench's refractive index difference as a function of radial position with respect to the outer cladding expressed in percentage, r_(t) is the outer radius of the depressed trench, and r_(int) is the inner radius of the depressed trench (e.g., the outer radius r₂ of the inner cladding). Those of ordinary skill in the art will appreciate that this equation can be used if the depressed trench's refractive index difference varies (i.e., the trench is non-rectangular) or if the refractive index difference is constant (i.e., the depressed trench is rectangular).

If the refractive index difference between the depressed trench and the outer cladding is constant, the volume v_(t) of the depressed trench can also be defined by the following equation:

v _(t)=Δ_(t)%×π×(r _(t) ² −r _(int) ²)

in which Δ_(t)% is the depressed trench's refractive index difference with respect to the outer cladding expressed in percentage, r_(t) is the outer radius of the depressed trench, and r_(int) is the inner radius of the depressed trench (e.g., the outer radius r₂ of the inner cladding).

The volume v_(t) of the depressed trench is typically between about 200%·μm² and 1,200%·μm². More typically, the volume v_(t) of the depressed trench is between about 250%·μm² and 750%·μm². The characteristics of the depressed trench facilitate the achievement of low bending losses.

The refractive index difference Δn_(t) between the depressed trench and the outer cladding and the width w_(t) of the depressed trench should be high enough to facilitate low bending losses. Moreover, the width w₂ of the inner cladding and the refractive index difference Δn_(t) between the depressed trench and the outer cladding facilitate the achievement of high bandwidths with a reduced cladding effect.

In this regard, the width w₂ of the inner cladding and the refractive index difference Δn_(t) between the depressed trench and the outer cladding typically satisfy the following inequality:

${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$

This relation between the width w₂ of the inner cladding and the refractive index difference Δn_(t) of the depressed trench makes it possible to achieve both low bending losses and a high bandwidth.

For example, at a wavelength of 850 nanometers, optical fibers in accordance with the present invention typically have: (i) for two turns with a bend radius (e.g., a radius of curvature) of 15 millimeters, bending losses of less than about 0.1 dB (e.g., less than about 0.05 dB), (ii) for two turns with a bend radius of 10 millimeters, bending losses of less than about 0.3 dB (e.g., less than about 0.1 dB), (iii) for two turns with a bend radius of 7.5 millimeters, bending losses of less than about 0.4 dB (e.g., less than about 0.2 dB), and (iv) for two turns with a bend radius of 5 millimeters, bending losses of less than about 1 dB (e.g., less than about 0.3 dB).

The present optical fibers have improved bandwidth in comparison with conventional fiber designs. In particular, at a wavelength of 850 nanometers, the present optical fibers typically have an OFL bandwidth of greater than about 5,000 MHz·km (e.g., greater than about 8,000 MHz·km), more typically higher than about 10,000 MHz·km (e.g., at least about 12,000 MHz·km). In an exemplary embodiment, at a wavelength of 850 nanometers, the present optical fibers have an OFL bandwidth of at least about 15,000 MHz·km (e.g., about 18,000 MHz·km or more).

The foregoing notwithstanding, OFL bandwidth is not the only parameter that can be used to evaluate an optical fiber's suitability for high-data-rate applications. In this regard, limiting the cladding effect in the optical fiber facilitates improved fiber performance in high-data-rate applications.

The cladding effect of an optical fiber may be determined by establishing the optical fiber's Radial Offset Bandwidth (ROB). The ROB is typically determined using DMD measurements, which are obtained by injecting an input pulse having (i) a wavelength of 850 nanometers and (ii) a spatial width of 5 microns+/−0.5 micron. Typically, the input pulse is obtained by coupling a light source (e.g., a semiconductor or titanium-sapphire laser) to a single-mode optical fiber having its outlet face positioned 10 microns or less from the inlet face of the multimode optical fiber. The temporal profile of the output pulse (i.e., the light pulse emitted from the outlet end of the multimode optical fiber) can be measured for each radial offset. The ROB at a radial offset X (in microns), denoted ROBX, is calculated by utilizing the information contained in the broadening and the deformation of the temporal profile of the output pulse obtained for an injection at the radial offset X for a given wavelength λ₀ (e.g., 850 nanometers), which corresponds to a given frequency f. A transfer function H^(X)(f) may be obtained using a Fourier transform and a pulse deconvolution corresponding to each radial offset.

In this regard, S_(e)(f) represents the Fourier transform of the input pulse measured according to the TIA-455-220-A 5.1 standard, which is hereby incorporated by reference in its entirety. Similarly, S_(s)(f,X) represents the Fourier transform of the output pulse corresponding to the X offset launch measured according to the TIA-455-220-A 5.1 standard. Those having ordinary skill in the art will recognize that the Fourier transform of the outlet pulse is a function of both frequency f and radial offset X.

For each offset launch X, a transfer function H^(X)(f) can be defined as follows:

${H^{X}(f)} = {\frac{S_{S}\left( {f,X} \right)}{S_{e}(f)}.}$

ROBX is the −3 dB bandwidth of the transfer function H^(X)(f) corresponding to the response of the optical fiber for an injection at a radial offset of X in the DMD measurements.

In practice, the bandwidth is calculated for an attenuation of −1.5 dB and then extrapolated for an attenuation of −3 dB, assuming a Gaussian response, and multiplied by a factor of √2 (as is also the case for the calculation of the effective bandwidth):

ROBX=√{square root over (2)}·f _(x)

and

10·log₁₀(H ^((X))(f _(x)))=−1.5.

An exemplary method of measuring DMD and calculating the effective modal bandwidth can be found in the FOTP-220 standard, which, as noted, is incorporated by reference in its entirety.

The radial offset bandwidth at a radial distance of 24 microns from the center of the optical fiber's central core (i.e., the ROB24 parameter) provides a good characterization of the cladding effect. A high ROB24 is indicative of a reduced cladding effect.

Optical fibers in accordance with the present invention typically exhibit a reduced cladding effect. In particular, optical fibers in accordance with the present invention typically have an ROB24, at a wavelength of 850 nanometers, of at least about 5,000 MHz·km (e.g., at least about 7,000 MHz·km, such as about 9,000 MHz·km or higher), more typically about 10,000 MHz·km or higher (e.g., at least about 12,500 MHz·km). In an exemplary embodiment, at a wavelength of 850 nanometers, the present optical fibers have an ROB24 of at least about 15,000 MHz·km (e.g., about 20,000 MHz·km or more).

FIG. 3 graphically depicts simulated ROB24 measurements of an exemplary optical fiber, at a wavelength of 850 nanometers, as function of the inner cladding's width w₂ and the refractive index difference Δn_(t) between the depressed trench and the outer cladding. The left-hand y-axis represents the inner cladding's width w₂. The x-axis represents the depressed trench's refractive index difference Δn_(t). The values for ROB24 corresponding to a (w₂, Δn_(t)) pair are shown in shades of gray. The darkest shade corresponds to an ROB24 of 2 GHz·km (i.e., 2,000 MHz·km), and the lightest shaded value corresponds to an ROB24 of 20 GHz·km (i.e., 20,000 MHz·km).

The highest ROB24 values are situated above curve a and below curve b. Curves a and b are defined by the following functions:

${{{curve}\mspace{14mu} a\text{:}\mspace{14mu} w_{2}} = {\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03}};{and}$ ${{curve}\mspace{14mu} b\text{:}\mspace{14mu} w_{2}} = {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$

Optical fibers that possess a pair of values (w₂, Δn_(t)) that are between the curves a and b will typically exhibit an ROB24 that is higher than 5,000 MHz·km (e.g., higher than 10,000 MHz·km). Thus, the present optical fibers exhibit minimized cladding effect and are suitable for use in high data rate applications.

An optical fiber's cladding effect may also be evaluated using differential-mode-delay measurements acquired with an outer mask. For example, the differential mode delay value on the outer mask (i.e., the outer DMD) can be obtained using the method of the FOTP-220 standard. For optical fibers with a core diameter of 50±3 microns (i.e., a core radius r₁ of between 23.5 microns and 26.5 microns) the outer DMD is measured using an outer mask of 0-23 microns. In other words, a differential mode delay value on the outer mask of 0-23 microns is measured using the DMD method over the radial offset range from the center of the central core (i.e., 0 microns) to 23 microns. Thus, signals coming from a radial offset of greater than 23 microns are ignored (e.g., between 23 and 25 microns for a core having a radius of 25 microns). Those of ordinary skill in the art will recognize that the dimensions of an outer mask may be modified for optical fibers having larger or smaller core diameters. For example, a mask with larger dimensions (e.g., a larger inner and outer radius) might be used with respect to a multimode optical fiber having a 62.5-micron diameter core. Similarly, a mask with smaller dimensions (e.g., a smaller inner and outer radius) might be used with respect to a multimode optical fiber having a core that is less than 50 microns.

The outer DMD originates from a plot for DMD measured over 750 meters of fiber. The light source used is typically a pulsed Ti:Sapphire laser emitting at 850 nanometers. The source emits pulses of less than 40 picoseconds at quarter height, and the RMS (Root Mean Square) spectral width is less than 0.1 nanometer.

An exemplary optical fiber according to the present invention exhibits improved outer DMD delay. In particular, at a wavelength of 850 nanometers, the exemplary optical fiber typically exhibits an outer DMD delay value of less than about 0.33 ps/m (e.g., less than 0.25 ps/m, such as 0.14 ps/m or less).

The advantages of the present invention will be more evident by contrasting comparative optical fibers with an exemplary optical fiber according to the present invention. Table 1 (below) shows fiber-profile-parameters of an exemplary optical fiber and two comparative optical fibers. Examples 1 and 3 are comparative optical fibers. Example 2 is an exemplary optical fiber according to the present invention.

TABLE 1 Examples a (μm) Δn₁ (×10⁻³) w₂ (μm) Δn_(t) (×10⁻³) w_(t) (μm) 1 25 14 1.6 −3.5 3 2 25 14 1.4 −5.5 3 3 25 14 0.8 −6.5 3

The optical fibers depicted in Table 1 have a similar refractive index profile; however, the optical fibers have different values for the inner cladding's width w₂ and the depressed trench's refractive index difference Δn_(t). The second optical fiber (i.e., Example 2) has an inner cladding width w₂ and a refractive index difference Δn_(t) between the depressed trench and the outer cladding which comply with the inequality:

${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$

Thus, the optical fiber in Example 2 can achieve low bending losses and a high bandwidth.

Further aspects of the present invention are illustrated in FIGS. 4-5. FIG. 4 graphically depicts the bandwidth at a radial offset of 24 microns (ROB24) as a function of the alpha parameter of the central core's alpha-index profile for Table 1's optical fibers (i.e., Examples 1, 2, and 3). Similarly, FIG. 5 graphically depicts the OFL bandwidth as a function of the alpha parameter of the central core's alpha-index profile for Table 1's optical fibers (i.e., Examples 1, 2, and 3). Both the ROB24 and the OFL bandwidth were measured using a wavelength of 850 nanometers.

FIG. 4 shows that the exemplary optical fiber according to the present invention (i.e., Example 2) has an ROB24 higher than 5,000 MHz·km, while the comparative optical fibers have an ROB24 below 5,000 MHz·km. Moreover, optical fibers in accordance with the present invention can exhibit an ROB24 higher than 10,000 MHz·km, or even higher than 15,000 MHz·km (e.g., 20,000 MHz·km or higher).

FIG. 5 shows that the exemplary optical fiber according to the present invention (i.e., Example 2) can exhibit an OFL bandwidth that is higher than that of comparative optical fibers. In particular, the optical fiber in Example 2 demonstrates an OFL bandwidth higher than 15,000 MHz·km.

Therefore, the present optical fibers typically exhibit a reduced cladding effect.

In one embodiment, the present optical fibers comply with ITU-T Recommendation G.651.1, which is hereby incorporated by reference in its entirety. Such exemplary optical fibers have a core diameter of 50 microns and a numerical aperture of 0.2±0.015 (i.e., between 0.185 and 0.215).

In one particular embodiment, the present optical fibers are compliant with the OM3 standard. Thus, the optical fibers have: (i) at a wavelength of 850 nanometers, an effective modal bandwidth EMB higher than 2,000 MHz·km, (ii) at a wavelength of 850 nanometers, an outer DMD below 0.3 ps/m, (iii) at a wavelength of 850 nanometers, an OFL bandwidth higher than 1,500 MHz·km, and (iv) a numerical aperture of between 0.185 and 0.215.

In another particular embodiment, the present optical fibers are compliant with the OM4 standard. Thus, the optical fibers have: (i) at a wavelength of 850 nanometers, an effective modal bandwidth EMB higher than 4700 MHz·km, (ii) at a wavelength of 850 nanometers, an outer DMD of less than 0.14 ps/m, (iii) at a wavelength of 850 nanometers, an OFL bandwidth higher than 3,500 MHz·km, and (iv) a numerical aperture of between 0.185 and 0.215. As noted, the OM3 standard and the OM4 standard are incorporated by reference in their entirety.

In another aspect, the present invention embraces an optical-fiber system (e.g., a multimode optical system) that includes at least a portion of an optical fiber as disclosed herein in accordance with the invention. In particular, the optical system can exhibit a data rate of at least 10 Gb/s over at least 100 meters (e.g., 300 meters).

The present optical fibers may facilitate the reduction in overall optical-fiber diameter. As will be appreciated by those having ordinary skill in the art, a reduced-diameter optical fiber is cost-effective, requiring less raw material. Moreover, a reduced-diameter optical fiber requires less deployment space (e.g., within a buffer tube and/or fiber optic cable), thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an optical fiber with a primary coating (and an optional secondary coating and/or ink layer) typically has an outer diameter of between about 235 microns and about 265 microns (μm). The component glass fiber itself (i.e., the glass core and surrounding cladding layers) typically has a diameter of about 125 microns, such that the total coating thickness is typically between about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fiber typically has an outer diameter of about 125 microns. With respect to the optical fiber's surrounding coating layers, the primary coating typically has an outer diameter of between about 175 microns and about 195 microns (i.e., a primary coating thickness of between about 25 microns and 35 microns), and the secondary coating typically has an outer diameter of between about 235 microns and about 265 microns (i.e., a secondary coating thickness of between about 20 microns and 45 microns). Optionally, the present optical fiber may include an outermost ink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduced diameter (e.g., an outermost diameter between about 150 microns and 230 microns). In this alternative optical fiber configuration, the thickness of the primary coating and/or secondary coating is reduced, while the diameter of the component glass fiber is maintained at about 125 microns. (Those having ordinary skill in the art will appreciate that, unless otherwise specified, diameter measurements refer to outer diameters.)

By way of illustration, in such exemplary embodiments the primary coating layer may have an outer diameter of between about 135 microns and about 175 microns (e.g., about 160 microns), typically less than 165 microns (e.g., between about 135 microns and 150 microns), and usually more than 140 microns (e.g., between about 145 microns and 155 microns, such as about 150 microns).

Moreover, in such exemplary embodiments the secondary coating layer may have an outer diameter of between about 150 microns and about 230 microns (e.g., more than about 165 microns, such as 190-210 microns or so), typically between about 180 microns and 200 microns. In other words, the total diameter of the optical fiber is reduced to less than about 230 microns (e.g., between about 195 microns and 205 microns, and especially about 200 microns). By way of further illustration, an optical fiber may employ a secondary coating of about 197 microns at a tolerance of +/−5 microns (i.e., a secondary-coating outer diameter of between 192 microns to 202 microns). Typically, the secondary coating will retain a thickness of at least about 10 microns (e.g., an optical fiber having a reduced thickness secondary coating of between 15 microns and 25 microns).

In another alternative embodiment, the outer diameter of the component glass fiber may be reduced to less than 125 microns (e.g., between about 60 microns and 120 microns), perhaps between about 70 microns and 115 microns (e.g., about 80-110 microns). This may be achieved, for instance, by reducing the thickness of one or more cladding layers. As compared with the prior alternative embodiment, (i) the total diameter of the optical fiber may be reduced (i.e., the thickness of the primary and secondary coatings are maintained in accordance with the prior alternative embodiment) or (ii) the respective thicknesses of the primary and/or secondary coatings may be increased relative to the prior alternative embodiment (e.g., such that the total diameter of the optical fiber might be maintained).

By way of illustration, with respect to the former, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 110 microns and 150 microns (e.g., about 125 microns) and a secondary coating layer having an outer diameter of between about 130 microns and 190 microns (e.g., about 155 microns). With respect to the latter, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 120 microns and 140 microns (e.g., about 130 microns) and a secondary coating layer having an outer diameter of between about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make the resulting optical fiber more susceptible to microbending attenuation. That said, the advantages of further

reducing optical-fiber diameter may be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coating layers (e.g., a primary coating and a secondary coating). At least one of the coating layers—typically the secondary coating—may be colored and/or possess other markings to help identify individual fibers. Alternatively, a tertiary ink layer may surround the primary and secondary coatings.

The present optical fibers may be manufactured by drawing from final preforms.

A final preform may be manufactured by providing a primary preform with an outer overcladding layer (i.e., an overcladding process). The outer overcladding layer typically consists of doped or undoped, natural or synthetic, silica glass. Several methods are available for providing the outer overcladding layer.

In a first exemplary method, the outer overcladding layer may be provided by depositing and vitrifying natural or synthetic silica particles on the outer periphery of the primary preform under the influence of heat. Such a process is known, for example, from U.S. Pat. Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which is hereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded using a silica sleeve tube, which may or may not be doped. This sleeve tube may then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be applied via an Outside Vapor Deposition (OVD) method. Here, a soot layer is first deposited on the outer periphery of a primary preform, and then the soot layer is vitrified to form glass.

The primary preforms may be manufactured via outside vapor deposition techniques, such as Outside Vapor Deposition (OVD) and Vapor Axial Deposition (VAD). Alternatively, the primary preforms may be manufactured via inside deposition techniques in which glass layers are deposited on the inner surface of a substrate tube of doped or undoped silica glass, such as Modified Chemical Vapor Deposition (MCVD), Furnace Chemical Vapor Deposition (FCVD), and Plasma Chemical Vapor Deposition (PCVD).

By way of example, the primary preforms may be manufactured using a PCVD process, which can precisely control the central core's gradient refractive index profile.

A depressed trench, for instance, may be deposited on the inner surface of a substrate tube as part of the chemical vapor deposition process. More typically, a depressed trench may be manufactured either (i) by using a fluorine-doped substrate tube as the starting point of the internal deposition process for deposition of the gradient refractive index central core or (ii) by sleeving a fluorine-doped silica tube over the gradient refractive index central core, which itself may be produced using an outside deposition process (e.g., OVD or VAD). Accordingly, a component glass fiber manufactured from the resulting preform may have a depressed trench located at the periphery of its central core.

As noted, a primary preform may be manufactured via an inside deposition process using a fluorine-doped substrate tube. The resulting tube containing the deposited layers may be sleeved by one or more additional fluorine-doped silica tubes so as to increase the thickness of a depressed trench, or to create a depressed trench having a varying refractive index over its width. Although not required, one or more additional sleeve tubes (e.g., fluorine-doped substrate tubes) may be collapsed onto the primary preform before an overcladding step is carried out. The process of sleeving and collapsing is sometimes referred to as jacketing and may be repeated to build several glass layers on the outside of the primary preform.

The present optical fibers may be deployed in various structures, such as those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosed within a buffer tube. For instance, optical fiber may be deployed in either a single-fiber loose buffer tube or a multi-fiber loose buffer tube. With respect to the latter, multiple optical fibers may be bundled or stranded within a buffer tube or other structure. In this regard, within a multi-fiber loose buffer tube, fiber sub-bundles may be separated with binders (e.g., each fiber sub-bundle is enveloped in a binder). Moreover, fan-out tubing may be installed at the termination of such loose buffer tubes to directly terminate loose buffered optical fibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermost optical fiber coating (i.e., tight buffered fiber) or otherwise surround the outermost optical-fiber coating or ink layer to provide an exemplary radial clearance of between about 50 and 100 microns (i.e., a semi-tight buffered fiber).

With respect to the former tight buffered fiber, the buffering may be formed by coating the optical fiber with a curable composition (e.g., a UV-curable material) or a thermoplastic material. The outer diameter of tight buffer tubes, regardless of whether the buffer tube is formed from a curable or non-curable material, is typically less than about 1,000 microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may be included between the optical fiber and the buffer tube (e.g., to provide a gliding layer).

As will be known by those having ordinary skill in the art, an exemplary buffer tube enclosing optical fibers as disclosed herein may be formed of polyolefins (e.g., polyethylene or polypropylene), including fluorinated polyolefins, polyesters (e.g., polybutylene terephthalate), polyamides (e.g., nylon), as well as other polymeric materials and blends. In general, a buffer tube may be formed of one or more layers. The layers may be homogeneous or include mixtures or blends of various materials within each layer.

In this context, the buffer tube may be extruded (e.g., an extruded polymeric material) or pultruded (e.g., a pultruded, fiber-reinforced plastic). By way of example, the buffer tube may include a material to provide high temperature and chemical resistance (e.g., an aromatic material or polysulfone material).

Although buffer tubes typically have a circular cross section, buffer tubes alternatively may have an irregular or non-circular shape (e.g., an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply be surrounded by an outer protective sheath or encapsulated within a sealed metal tube. In either structure, no intermediate buffer tube is necessarily required.

Multiple optical fibers as disclosed herein may be sandwiched, encapsulated, and/or edge bonded to form an optical fiber ribbon. Optical fiber ribbons can be divisible into subunits (e.g., a twelve-fiber ribbon that is splittable into six-fiber subunits). Moreover, a plurality of such optical fiber ribbons may be aggregated to form a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or a ribbon stack in which the uppermost and lowermost optical fiber ribbons have fewer optical fibers than those toward the center of the stack. This construction may be useful to increase the density of optical elements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmission elements in buffer tubes or cables, subject to other constraints (e.g., cable or mid-span attenuation). The optical elements themselves may be designed for increased packing density. For example, the optical fiber may possess modified properties, such as improved refractive-index profile, core or cladding dimensions, or primary-coating thickness and/or modulus, to improve microbending and macrobending characteristics.

By way of example, a rectangular ribbon stack may be formed with or without a central twist (i.e., a “primary twist”). Those having ordinary skill in the art will appreciate that a ribbon stack is typically manufactured with rotational twist to allow the tube or cable to bend without placing excessive mechanical stress on the optical fibers during winding, installation, and use. In a structural variation, a twisted (or untwisted) rectangular ribbon stack may be further formed into a coil-like configuration (e.g., a helix) or a wave-like configuration (e.g., a sinusoid). In other words, the ribbon stack may possess regular “secondary” deformations.

As will be known to those having ordinary skill in the art, such optical fiber ribbons may be positioned within a buffer tube or other surrounding structure, such as a buffer-tube-free cable. Subject to certain restraints (e.g., attenuation), it is desirable to increase the density of elements such as optical fibers or optical fiber ribbons within buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be positioned externally adjacent to and stranded around a central strength member. This stranding can be accomplished in one direction, helically, known as “S” or “Z” stranding, or Reverse Oscillated Lay stranding, known as “S-Z” stranding. Stranding about the central strength member reduces optical fiber strain when cable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit of minimizing fiber strain for both tensile cable strain and longitudinal compressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur during installation, the cable will become longer while the optical fibers can migrate closer to the cable's neutral axis to reduce, if not eliminate, the strain being translated to the optical fibers. With respect to longitudinal compressive strain, which may occur at low operating temperatures due to shrinkage of the cable components, the optical fibers will migrate farther away from the cable's neutral axis to reduce, if not eliminate, the compressive strain being translated to the optical fibers.

In a variation, two or more substantially concentric layers of buffer tubes may be positioned around a central strength member. In a further variation, multiple stranding elements (e.g., multiple buffer tubes stranded around a strength member) may themselves be stranded around each other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be simply placed externally adjacent to the central strength member (i.e., the buffer tubes are not intentionally stranded or arranged around the central strength member in a particular manner and run substantially parallel to the central strength member).

Alternatively still, the present optical fibers may be positioned within a central buffer tube (i.e., the central buffer tube cable has a central buffer tube rather than a central strength member). Such a central buffer tube cable may position strength members elsewhere. For instance, metallic or non-metallic (e.g., GRP) strength members may be positioned within the cable sheath itself, and/or one or more layers of high-strength yarns (e.g., aramid or non-aramid yarns) may be positioned parallel to or wrapped (e.g., contrahelically) around the central buffer tube (i.e., within the cable's interior space). As will be understood by those having ordinary skill in the art, such strength yarns provide tensile strength to fiber optic cables. Likewise, strength members can be included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers), which may reduce unwanted attenuation in fiber optic cables (e.g., rectangular, flat ribbon cables or round, loose tube cables) that are subjected to relatively tight bends (i.e., a low bend radius). Moreover, the presence of a lubricant on strength yarns (e.g., aramid strength yarns) may facilitate removal of the cable jacketing by reducing unwanted bonding between the strength yarns and the surrounding cable jacket.

In other embodiments, the optical fibers may be placed within a slotted core cable. In a slotted core cable, optical fibers, individually or as a fiber ribbon, may be placed within pre-shaped helical grooves (i.e., channels) on the surface of a central strength member, thereby forming a slotted core unit. The slotted core unit may be enclosed by a buffer tube. One or more of such slotted core units may be placed within a slotted core cable. For example, a plurality of slotted core units may be helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitube cable design, whereby the optical fibers are stranded around themselves within a large multi-fiber loose buffer tube rather than around a central strength member. In other words, the large multi-fiber loose buffer tube is centrally positioned within the maxitube cable. For example, such maxitube cables may be deployed in optical ground wires (OPGW).

In another cabling embodiment, multiple buffer tubes may be stranded around themselves without the presence of a central member. These stranded buffer tubes may be surrounded by a protective tube. The protective tube may serve as the outer casing of the fiber optic cable or may be further surrounded by an outer sheath. The protective tube may tightly or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additional elements may be included within a cable core. For example, copper cables or other active, transmission elements may be stranded or otherwise bundled within the cable sheath. Passive elements may also be placed within the cable core, such as between the interior walls of the buffer tubes and the enclosed optical fibers. Alternatively and by way of example, passive elements may be placed outside the buffer tubes between the respective exterior walls of the buffer tubes and the interior wall of the cable jacket, or within the interior space of a buffer-tube-free cable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or other materials containing water-swellable material and/or coated with water-swellable materials (e.g., including super absorbent polymers (SAPs), such as SAP powder) may be employed to provide water blocking and/or to couple the optical fibers to the surrounding buffer tube and/or cable jacketing (e.g., via adhesion, friction, and/or compression). Exemplary water-swellable elements are disclosed in commonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which is hereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive, such as a silicone acrylate cross-linked by exposure to actinic radiation) may be provided on one or more passive elements (e.g., water-swellable material) to bond the elements to the buffer tube. An adhesive material may also be used to bond the water-swellable element to optical fibers within the buffer tube. Exemplary arrangements of such elements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which is hereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain a thixotropic composition (e.g., grease or grease-like gels) between the optical fibers and the interior walls of the buffer tubes. For example, filling the free space inside a buffer tube with water-blocking, petroleum-based filling grease helps to block the ingress of water. Further, the thixotropic filling grease mechanically (i.e., viscously) couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, thereby hindering connection and splicing operations. Thus, the present optical fibers may be deployed in dry cable structures (i.e., grease-free buffer tubes).

Exemplary buffer tube structures that are free from thixotropic filling greases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for a Coupling Composition for Optical Fiber Cables (Parris et al.), which is hereby incorporated by reference in its entirety. Such buffer tubes employ coupling compositions formed from a blend of high-molecular weight elastomeric polymers (e.g., about 35 weight percent or less) and oils (e.g., about 65 weight percent or more) that flow at low temperatures. Unlike thixotropic filling greases, the coupling composition (e.g., employed as a cohesive gel or foam) is typically dry and, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cable enclosing optical fibers as disclosed herein may have a sheath formed from various materials in various designs. Cable sheathing may be formed from polymeric materials such as, for example, polyethylene, polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon), polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylene propylene, polyvinyl fluoride, or polyvinylidene difluoride), and ethylene vinyl acetate. The sheath and/or buffer tube materials may also contain other additives, such as nucleating agents, flame-retardants, smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectric material (e.g., non-conducting polymers), with or without supplemental structural components that may be used to improve the protection (e.g., from rodents) and strength provided by the cable sheath. For example, one or more layers of metallic (e.g., steel) tape, along with one or more dielectric jackets, may form the cable sheathing. Metallic or fiberglass reinforcing rods (e.g., GRP) may also be incorporated into the sheath. In addition, aramid, fiberglass, or polyester yarns may be employed under the various sheath materials (e.g., between the cable sheath and the cable core), and/or ripcords may be positioned, for example, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have a circular cross section, but cable sheaths alternatively may have an irregular or non-circular shape (e.g., an oval, trapezoidal, or flat cross-section).

By way of example, the present optical fiber may be incorporated into single-fiber drop cables, such as those employed for Multiple Dwelling Unit (MDU) applications. In such deployments, the cable jacketing must exhibit crush resistance, abrasion resistance, puncture resistance, thermal stability, and fire resistance as required by building codes. An exemplary material for such cable jackets is thermally stable, flame-retardant polyurethane (PUR), which mechanically protects the optical fibers yet is sufficiently flexible to facilitate easy MDU installations. Alternatively, a flame-retardant polyolefin or polyvinyl chloride sheath may be used.

In general, and as will be known to those having ordinary skill in the art, a strength member is typically in the form of a rod or braided/helically wound wires or fibers, though other configurations will be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may be variously deployed, including as drop cables, distribution cables, feeder cables, trunk cables, and stub cables, each of which may have varying operational requirements (e.g., temperature range, crush resistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts, plenums, or risers. By way of example, an optical fiber cable may be installed in an existing duct or microduct by pulling or blowing (e.g., using compressed air). An exemplary cable installation method is disclosed in commonly assigned U.S. Pat. No. 7,574,095 for a Communication Cable Assembly and Installation Method, (Lock et al.), and U.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized Communication Cable Assembly and Installation Method, (Griffioen et al.), each of which is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be stranded (e.g., around a central strength member). In such configurations, an optical fiber cable's protective outer sheath may have a textured outer surface that periodically varies lengthwise along the cable in a manner that replicates the stranded shape of the underlying buffer tubes. The textured profile of the protective outer sheath can improve the blowing performance of the optical fiber cable. The textured surface reduces the contact surface between the cable and the duct or microduct and increases the friction between the blowing medium (e.g., air) and the cable. The protective outer sheath may be made of a low coefficient-of-friction material, which can facilitate blown installation. Moreover, the protective outer sheath can be provided with a lubricant to further facilitate blown installation.

In general, to achieve satisfactory long-distance blowing performance (e.g., between about 3,000 to 5,000 feet or more), the outer cable diameter of an optical fiber cable should be no more than about 70 to 80 percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an air blown fiber system. In an air blown fiber system, a network of unfilled cables or microducts is installed prior to the installation of optical fibers. Optical fibers may subsequently be blown into the installed cables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the ground or, as an aerial cable, suspended from a pole or pylon. An aerial cable may be self-supporting, or secured or lashed to a support (e.g., messenger wire or another cable). Exemplary aerial fiber optic cables include overhead ground wires (OPGW), all-dielectric self-supporting cables (ADSS), all dielectric lash cables (AD-Lash), and figure-eight cables, each of which is well understood by those having ordinary skill in the art. Figure-eight cables and other designs can be directly buried or installed into ducts, and may optionally include a toning element, such as a metallic wire, so that they can be found with a metal detector.

In addition, although the optical fibers may be further protected by an outer cable sheath, the optical fiber itself may be further reinforced so that the optical fiber may be included within a breakout cable, which allows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmission system, connections are required at various points in the network. Optical fiber connections are typically made by fusion splicing, mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber ends either in the field (e.g., at the network location) or in a factory prior to installation into the network. The ends of the connectors are mated in the field in order to connect the optical fibers together or connect the optical fibers to the passive or active components. For example, certain optical fiber cable assemblies (e.g., furcation assemblies) can separate and convey individual optical fibers from a multiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplemental equipment, which itself may employ the present optical fiber as previously disclosed. For instance, an amplifier may be included to improve optical signals. Dispersion compensating modules may be installed to reduce the effects of chromatic dispersion and polarization mode dispersion. Splice boxes, pedestals, and distribution frames, which may be protected by an enclosure, may likewise be included. Additional elements include, for example, remote terminal switches, optical network units, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use in a communication system (e.g., networking or telecommunications). A communication system may include fiber optic cable architecture such as fiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure (FTTE), fiber-to-the-curb (FITC), fiber-to-the-building (FTTB), and fiber-to-the-home (FTTH), as well as long-haul or metro architecture. Moreover, an optical module or a storage box that includes a housing may receive a wound portion of the optical fiber disclosed herein. By way of example, the optical fiber may be wound with a bending radius of less than about 15 millimeters (e.g., 10 millimeters or less, such as about 5 millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications, including, without limitation, fiber optic sensors or illumination applications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). As will be known by those having ordinary skill in the art, FBG is a periodic or aperiodic variation in the refractive index of an optical fiber core and/or cladding. This variation in the refractive index results in a range of wavelengths (e.g., a narrow range) being reflected rather than transmitted, with maximum reflectivity occurring at the Bragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber by exposing the optical fiber to an intense source of ultraviolet light (e.g., a UV laser). In this respect, UV photons may have enough energy to break molecular bonds within an optical fiber, which alters the structure of the optical fiber, thereby increasing the optical fiber's refractive index. Moreover, dopants (e.g., boron or germanium) and/or hydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation of FBG, the coating may be removed. Alternatively, coatings that are transparent at the particular UV wavelengths (e.g., the UV wavelengths emitted by a UV laser to write FBG) may be employed to render coating removal unnecessary. In addition, silicone, polyimide, acrylate, or PFCB coatings, for instance, may be employed for high-temperature applications.

A particular FBG pattern may be created by employing (i) a photomask placed between the UV light source and the optical fiber, (ii) interference between multiple UV light beams, which interfere with each other in accordance with the desired FBG pattern (e.g., a uniform, chirped, or titled pattern), or (iii) a narrow UV light beam for creating individual variations. The FBG structure may have, for example, a uniform positive-only index change, a Gaussian-apodized index change, a raised-cosine-apodized index change, or a discrete phase shift index change. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensing applications (e.g., for detecting vibration, temperature, pressure, moisture, or movement). In this respect, changes in the optical fiber (e.g., a change in temperature) result in a shift in the Bragg wavelength, which is measured by a sensor. FBG may be used to identify a particular optical fiber (e.g., if the optical fiber is broken into pieces).

Fiber Bragg Grating may also be used in various active or passive communication components (e.g., wavelength-selective filters, multiplexers, demultiplexers, Mach-Zehnder interferometers, distributed Bragg reflector lasers, pump/laser stabilizers, and supervisory channels).

To supplement the present disclosure, this application incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 4,838,643 for a Single Mode Bend Insensitive Fiber for Use in Fiber Optic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 for a Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (de Montmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic Dispersion Compensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,555,186 for an Optical Fiber (Flammer et al.); U.S. Patent Application Publication No. US2009/0252469 A1 for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. patent application Ser. No. 12/098,804 for a Transmission Optical Fiber Having Large Effective Area (Sillard et al.), filed Apr. 7, 2008; International Patent Application Publication No. WO 2009/062131 A1 for a Microbend-Resistant Optical Fiber, (Overton); U.S. Patent Application Publication No. US2009/0175583 A1 for a Microbend-Resistant Optical Fiber, (Overton); U.S. Patent Application Publication No. US2009/0279835 A1 for a Single-Mode Optical Fiber Having Reduced Bending Losses, filed May 6, 2009, (de Montmorillon et al.); U.S. Patent Application Publication No. US2009/0279836 A1 for a Bend-Insensitive Single-Mode Optical Fiber, filed May 6, 2009, (de Montmorillon et al.); U.S. Patent Application Publication No. US2010/0021170 A1 for a Wavelength Multiplexed Optical System with Multimode Optical Fibers, filed Jun. 23, 2009, (Lumineau et al.); U.S. Patent Application Publication No. US2010/0028020 A1 for a Multimode Optical Fibers, filed Jul. 7, 2009, (Gholami et al.); U.S. Patent Application Publication No. US2010/0119202 A1 for a Reduced-Diameter Optical Fiber, filed Nov. 6, 2009, (Overton); U.S. Patent Application Publication No. US2010/0142969 A1 for a Multimode Optical System, filed Nov. 6, 2009, (Gholami et al.); U.S. Patent Application Publication No. US2010/0118388 A1 for an Amplifying Optical Fiber and Method of Manufacturing, filed Nov. 12, 2009, (Pastouret et al.); U.S. Patent Application Publication No. US2010/0135627 A1 for an Amplifying Optical Fiber and Production Method, filed Dec. 2, 2009, (Pastouret et al.); U.S. Patent Application Publication No. US2010/0142033 for an Ionizing Radiation-Resistant Optical Fiber Amplifier, filed Dec. 8, 2009, (Regnier et al.); U.S. Patent Application Publication No. US2010/0150505 A1 for a Buffered Optical Fiber, filed Dec. 11, 2009, (Testu et al.); U.S. Patent Application Publication No. US2010/0171945 for a Method of Classifying a Graded-Index Multimode Optical Fiber, filed Jan. 7, 2010, (Gholami et al.); U.S. Patent Application Publication No. US2010/0189397 A1 for a Single-Mode Optical Fiber, filed Jan. 22, 2010, (Richard et al.); U.S. Patent Application Publication No. US2010/0189399 A1 for a Single-Mode Optical Fiber Having an Enlarged Effective Area, filed Jan. 27, 2010, (Sillard et al.); U.S. Patent Application Publication No. US2010/0189400 A1 for a Single-Mode Optical Fiber, filed Jan. 27, 2010, (Sillard et al.); U.S. Patent Application Publication No. US2010/0214649 A1 for an Optical Fiber Amplifier Having Nanostructures, filed Feb. 19, 2010, (Burow et al.); U.S. Patent Application Publication No. US2010/0254653 A1 for a Multimode Fiber, filed Apr. 22, 2010, (Molin et al.); U.S. patent application Ser. No. 12/794,229 for a Large Bandwidth Multimode Optical Fiber Having a Reduced Cladding Effect, filed Jun. 4, 2010, (Molin et al.); U.S. patent application Ser. No. 12/878,449 for a Multimode Optical Fiber Having Improved Bending Losses, filed Sep. 9, 2010, (Molin et al.); U.S. patent application Ser. No. 12/884,834 for a Multimode Optical Fiber, filed Sep. 17, 2010, (Molin et al.); U.S. patent application Ser. No. 12/887,813 for an Optical Fiber for Sum-Frequency Generation, filed Sep. 22, 2010, (Richard et al.); and U.S. patent application Ser. No. ______ for a High-Bandwidth, Dual-Trench-Assisted Multimode Optical Fiber, filed Nov. 24, 2010, (Molin et al.).

To supplement the present disclosure, this application further incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,717,805 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,911,023 for Polyolefin Materials Suitable for Optical Fiber Cable Components; U.S. Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No. 6,066,397 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon and Method for Making the Same; U.S. Pat. No. 6,085,009 for Water Blocking Gels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes and Cables Made Therewith; U.S. Pat. No. 6,215,931 for Flexible Thermoplastic Polyolefin Elastomers for Buffering Transmission Elements in a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method for Accessing Optical Fibers in the Midspan Region of an Optical Fiber Cable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbon and Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method for Accessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224 for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section; U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix Material Having Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for an Optical Fiber Having Water Swellable Material for Identifying Grouping of Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for Manufacturing Optical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491 for an Optical Drop Cable for Aerial Installation; U.S. Pat. No. 7,346,244 for a Coated Central Strength Member for Fiber Optic Cables with Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skin for Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube that Results in Easy Access to and Low Attenuation of Fibers Disposed Within Buffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-Speed Gel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446 for an Optical Fiber Cable with Cushion Members Protecting Optical Fiber Ribbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No. 6,618,538 for a Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No. 7,322,122 for a Method and Apparatus for Curing a Fiber Having at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347 for an Optimized Fiber Optic Cable Suitable for Microduct Blown Installation; U.S. Pat. No. 6,941,049 for a Fiber Optic Cable Having No Rigid Strength Members and a Reduced Coefficient of Thermal Expansion; U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to Prevent Fiber Retraction; U.S. Pat. No. 7,515,795 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton et al.); U.S. Patent Application Publication No. 2008/0292262 for a Grease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing a Water-Swellable, Texturized Yarn (Overton et al.); European Patent Application Publication No. 1,921,478 A1, for a Telecommunication Optical Fiber Cable (Tatat et al.); U.S. Pat. No. 7,702,204 for a Method for Manufacturing an Optical Fiber Preform (Gonnet et al.); U.S. Pat. No. 7,570,852 for an Optical Fiber Cable Suited for Blown Installation or Pushing Installation in Microducts of Small Diameter (Nothofer et al.); U.S. Pat. No. 7,526,177 for a Fluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No. 7,646,954 for an Optical Fiber Telecommunications Cable (Tatat); U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element (Overton et al.); U.S. Pat. No. 7,567,739 for a Fiber Optic Cable Having a Water-Swellable Element (Overton); U.S. Pat. No. 7,817,891 for a Method for Accessing Optical Fibers within a Telecommunication Cable (Lavenne et al.); U.S. Pat. No. 7,639,915 for an Optical Fiber Cable Having a Deformable Coupling Element (Parris et al.); U.S. Pat. No. 7,646,952 for an Optical Fiber Cable Having Raised Coupling Supports (Parris); U.S. Pat. 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In the specification and/or figures, typical embodiments of the invention have been disclosed. The present invention is not limited to such exemplary embodiments. The use of the term “and/or” includes any and all combinations of one or more of the associated listed items. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation. 

1. A multimode optical fiber, comprising: a central core surrounded by an outer cladding, said central core having a radius r₁ and an alpha-index profile with respect to said outer cladding; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to said outer cladding; and a depressed trench positioned between said inner cladding and said outer cladding, said depressed trench having (i) an outer radius r_(t), (ii) a width w_(t), and (iii) a refractive index difference Δn_(t) with respect to said outer cladding; wherein said inner cladding's width w₂ and said depressed trench's refractive index difference Δn_(t) satisfy the following inequality: ${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$
 2. A multimode optical fiber according to claim 1, wherein said central core's alpha-index profile has an alpha parameter α of between about 1.9 and 2.1.
 3. A multimode optical fiber according to claim 1, wherein, with respect to said outer cladding, said central core has a maximum refractive index difference Δn_(t) of between about 11×10⁻³ and 16×10⁻³.
 4. A multimode optical fiber according to claim 1, wherein said inner cladding immediately surrounds said central core and said depressed trench immediately surrounds said inner cladding.
 5. A multimode optical fiber according to claim 1, wherein said inner cladding's refractive index difference Δn₂ is between about −0.05×10⁻³ and 0.05×10⁻³.
 6. A multimode optical fiber according to claim 1, wherein said depressed trench's refractive index difference Δn_(t) is between about −15×10⁻³ and −3×10⁻³.
 7. A multimode optical fiber according to claim 1, wherein said depressed trench's width w_(t) is between about three microns and five microns.
 8. A multimode optical fiber according to claim 1, wherein said depressed trench has a volume v_(t) of between about 200%·μm² and 1,200%·μm².
 9. A multimode optical fiber according to claim 1, wherein, for two turns around a bend radius of 15 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.1 dB.
 10. A multimode optical fiber according to claim 1, wherein, for two turns around a bend radius of 10 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.3 dB.
 11. A multimode optical fiber according to claim 1, wherein, for two turns around a bend radius of 7.5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.2 dB.
 12. A multimode optical fiber according to claim 1, wherein, for two turns around a bend radius of 5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.3 dB.
 13. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 5,000 MHz·km.
 14. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 10,000 MHz·km.
 15. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 15,000 MHz·km.
 16. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of at least about 1,500 MHz·km.
 17. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of at least about 3,500 MHz·km.
 18. A multimode optical fiber according to claim 1, wherein the multimode optical fiber has a numerical aperture of 0.2±0.015.
 19. A multimode optical fiber according to claim 1, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has an outer DMD value of less than about 0.33 ps/m.
 20. A multimode optical fiber according to claim 1, wherein: said central core's radius r₁ is 25 microns±1.5 microns; and at a wavelength of 850 nanometers, the multimode optical fiber has an outer DMD value (0-23 microns) of less than about 0.14 ps/m.
 21. An optical-fiber system comprising at least a portion of the multimode optical fiber according to claim
 1. 22. An optical-fiber system according to claim 21, wherein the optical-fiber system has a data rate of at least about 10 Gb/s over a distance of about 100 meters.
 23. An optical-fiber system according to claim 21, wherein the optical-fiber system has a data rate of at least about 10 Gb/s over a distance of about 300 meters.
 24. A multimode optical fiber, comprising: a central core surrounded by an outer cladding, said central core having a radius r₁ and an alpha-index profile with respect to said outer cladding, the alpha-index profile having an alpha parameter α of between about 1.9 and 2.1; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to said outer cladding; and a depressed trench positioned between said inner cladding and said outer cladding, said depressed trench having (i) an outer radius r_(t), (ii) a width w_(t), (iii) a refractive index difference Δn_(t) with respect to said outer cladding, and (iv) a volume v_(t) of between about 250%·μm² and 750%·μm²; wherein said inner cladding's width w₂ and said depressed trench's refractive index difference Δn_(t) satisfy the following inequality: ${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$
 25. A multimode optical fiber according to claim 24, wherein, for two turns around a bend radius of 15 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.05 dB.
 26. A multimode optical fiber according to claim 24, wherein, for two turns around a bend radius of 7.5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.4 dB.
 27. A multimode optical fiber according to claim 24, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 7,000 MHz·km.
 28. A multimode optical fiber according to claim 24, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of at least about 5,000 MHz·km.
 29. A multimode optical fiber according to claim 24, wherein: said central core's radius r₁ is between about 23.5 microns and 26.5 microns; and at a wavelength of 850 nanometers, the multimode optical fiber has an outer DMD value (0-23 microns) of less than about 0.25 ps/m.
 30. A multimode optical fiber, comprising: a central core surrounded by an outer cladding, said central core having (i) a radius r₁, (ii) an alpha-index profile with respect to said outer cladding, and (iii) a maximum refractive index difference Δn₁ with said outer cladding of between about 11×10⁻³ and 16×10⁻³; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with said outer cladding of between about −0.05×10⁻³ and 0.05×10⁻³; and a depressed trench positioned between said inner cladding and said outer cladding, said depressed trench having (i) an outer radius r_(t), (ii) a width w_(t), and (iii) a refractive index difference Δn_(t) with said outer cladding of between about −15×10⁻³ and −3×10⁻³; wherein said inner cladding's width w₂ and said depressed trench's refractive index difference Δn_(t) satisfy the following inequality: ${\frac{5.6}{{1000 \times \Delta \; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{1000 \times \Delta \; n_{t}} - 0.4} + 2.}$
 31. A multimode optical fiber according to claim 30, wherein said central core's alpha-index profile has an alpha parameter α of between about 2.04 and 2.1.
 32. A multimode optical fiber according to claim 30, wherein said depressed trench has a volume v_(t) of between about 250%·μm² and 750%·μm².
 33. A multimode optical fiber according to claim 30, wherein, for two turns around a bend radius of 10 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.1 dB.
 34. A multimode optical fiber according to claim 30, wherein, for two turns around a bend radius of 5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 1 dB.
 35. A multimode optical fiber according to claim 30, wherein, at a wavelength of 850 nanometers, the multimode optical fiber has a radial offset bandwidth at 24 microns (ROB24) of at least about 5,000 MHz·km. 